Oil lubricated rolling element bearings or sleeve bearings are widely used to support rotors in various types of turbomachinery due to their large load capacity and predictable performance. However, oil lubrication circuits make these systems complicated and environmentally unfriendly. Another type of bearing, the traditional hydrostatic air bearing with solid circular wall (without bump foil and top foil) is widely used in machine tool applications. However, the traditional hydrostatic air bearings have a limited capability to accommodate rotor-bearing misalignment, debris, and contaminations.
In contrast to traditional hydrostatic air bearings, air foil bearings (also referred to as gas foil bearings) have large load capacity (at high speeds) and can tolerate rotor-bearing misalignment and debris. Moreover, in midsized turbomachinery, air foil bearings offer an alternative solution to the oil lubricated bearings because the systems are light and compact, and can improve reliability. Air foil bearings not only offer very low friction during operation but also circumvent the need of oil lubrication circuits, seals, and oil cooling system, allowing the system to be less complicated and more environment-friendly. Accordingly, air foil bearings are often used in auxiliary power units (APU), air management systems for aircraft, automobiles, micro-gas turbines (MGT) as independent power generators or for fuel cell-MGT hybrid systems, turbochargers, turbo compressors, and other applications.
Referring now to FIG. 1, a typical configuration of a bump foil air or gas bearing 100 is shown in accordance with the prior art. The air foil bearing 100 includes a smooth top foil 102 and a corrugated bump foil 104 disposed within a housing or bearing sleeve 106. The bump foil 104 sustains the applied load from the journal or shaft 108 and provides structural stiffness and damping. The bump foil 104 can also accommodate misalignments and distortions of the shaft 108. The top foil 102 and corrugated bump foil 104 are single pieces welded together at one end 110 such that the other end 112 is free to move. Note that other compliant structures can be used instead of the bump foil 104, such as multiple bump foils and overlapping top foils (U.S. Patent Application Publication No. 2002/0097927), multiple top foils with actuators (U.S. Pat. No. 6,582,125), etc. The structure of the compliant structure determines the overall performance and characteristics of the air foil bearing 100.
While it is stationary, the journal, rotor or shaft 108 sits on the top foil 102, forming a wedge shape between the top foil 102 and rotor 108. When the rotor 108 starts to spin, hydrodynamic pressure is generated from the wedge, lifting the rotor 108 to be completely airborne (air space 114). The bump foil 104 deforms elastically according to the generated pressure and friction of the bearing becomes very small. As a result, air foil bearings 100 can very effectively accommodate misalignment and rotor growth (both centrifugal and thermal). In addition, air foil bearings 100 can use any gas as a working and cooling fluid, there is no need for additional lubrication mechanisms, thus enabling the construction of a compact and maintenance free system.
Despite extensive research on air foil bearings 100 and successful applications to many oil-free turbomachinery at low to intermediate temperature ranges, successful implementation of air foil bearings 100 on high temperature applications such as small gas turbines are very few. The air foil bearings 100 have limited reliability issue that comes from dry rubbing during start/stops and limited heat dissipation capability. It can be assumed that the air has very small viscosity compared to oil, and thus cooling is not as critically important as oil-lubricated hydrodynamic bearings. However, very small clearance and high speed operation can generate fairly good amount of heat that cannot be neglected. Regardless of lubricating media, the hydrodynamic pressure provides only load-support but not dissipation of parasitic energy generated by viscous drag or heat input from other parts of the machine. It is well known that the purpose of continuous oil circulation in oil lubricated hydrodynamic bearings is to dissipate parasitic energy generated within/transferred to the bearing, and keep the oil temperature within certain limit so that the oil can generate necessary hydrodynamic pressure.
Very often critical advantages of air foil bearing 100 are claimed as no lubrication circuit and little friction (no heating), providing a “simple” environment-friendly solution. However, considering the two critical functions (load support and parasitic energy dissipation) of the bearings, the hydrodynamic air foil bearing 100 cannot be as simple as claimed. In fact, the success of bump foil bearings in industrial applications owes to continuous air cooling through the space behind the top foil 102 and adequate surface coatings that can survive dry rubbings during start/stops. The importance of cooling and wear-resistant coating is more evident when the hydrodynamic air foil bearings 100 are operated under large external loads.
One of the critical technical issues related to reliability of the air foil bearings 100 is wear on the top foil 102 and rotor 108 during start/stops. The hydrodynamic pressure is generated only when the rotor 108 reaches a certain speed. At start/stop and low speeds the hydrodynamic pressure is not large enough and rubbing/wear happens on the top foil 102 and rotor 108. The susceptibility to wear prevents widespread use of the air foil bearing 100 technology in large turbo machines. In large turbo machines, rotor 108 weight itself is similar to the load capacity of the bearing at high speeds. Therefore, at low speeds, the bearing do not have enough load capacity and suffer severe dry rubbing, wear, heat generation, and eventually failure. Bearing cooling is also mandatory for certain applications because the foil bearings can generate a significant amount of heat depending on the operating conditions. Usually axial flow is used through the space between the top foil 102 and bearing sleeve or housing 106.
The notorious thermal runaway of air foil bearings 100 comes from mainly rotor 108 thermal expansion/distortion rather than the bearing itself. Because the cooling air passes through the space behind the top foil 102, the cooling efficiency is low and can consume a lot of air to achieve required cooling performance. Furthermore, current cooling methods cannot be effective in controlling the rotor thermal distortion. As a result, another method to address the cooling limitations of hydrodynamic air foil bearings 100 and the dry rubbing issue was developed.
In small machines, the rotor weight does not impose significant load to the AFB during start/stops, and relevant dry rubbing is not critical issue compared to the rotor-bearing instability at high speeds. However, as machine size increases, the rotor weight increases in proportion to the third power of rotor characteristic dimension while the AFB's load capacity increases in proportion to the second power of rotor characteristic dimension. Therefore, hydrodynamic AFBs have a definitive load capacity limit as the machine size increases. Furthermore, the hydrodynamic AFBs with adequate load capacity at machine operating speed has to rely on boundary lubrication with surface coatings during start/stops, yielding inevitable surface wear and limited reliability. Much progress was reported on the surface coatings on the AFBs [1-4], and successful application of one of these coatings to small power generation turbines is reported in [5]. For example, NASA has been developing a PS 300 series ceramic metal composite coating for high temperature applications. Mohawk Innovative Technology has also developed a series of metal-ceramic composite coatings applied to the top foil 102 via a thermal spray process. Other low friction coatings are disclosed in U.S. Patent Application Publication Nos. 2007/0003693 and 2008/0057223. These types of coatings have shown proven performance at limited temperature ranges depending on the material composition. Because of the limited performance of these solid lubricants, the wear of the top foil 102 and rotor 108 is inevitable during the repeated start and stop cycles, and performance also degrades accordingly. However, in mid-sized turbomachinery where journal diameter is bigger than 100 mm (˜4 inch), the reliability issue associated with dry rubbing during start/stop seems unavoidable as presented in [6].
Hydrostatic gas bearings with cylindrical solid wall began to appear for military and space applications in 60's [7-9] to avoid the dry rubbing during start/stop. Other forms of hydrostatic gas bearings were reported since then; Curwen et al [10] reported tilting pad gas bearing with hydrostatic orifice formed on the spherical pivot for Brayton cycle turbo generator. Han et al [11] investigated dynamic performance of cylindrical hydrostatic air bearing with multiple inherent restrictors along the circumferential and axial directions for precision machine tool applications. More recently, hydrostatic operation was applied to three-lobed gas bearing [12] and flexure pivot tilting pad gas bearing [13, 14]. The concept of the hydrostatic lift can be found in a patent on foil thrust bearing [15]. However, in the patent [15], compressor bleed air is discharged to the backside of the top foil (i.e., space taken by bump foils) and then guided to the air film through multiple holes formed on the top foil. The concept is very questionable in terms of effectiveness because the pressure on the backside of the top foil is higher than hydrostatic pressure generated in the air film, resulting in no hydrostatic lift. Application of hydrostatic bearing concept can be also found in a sheet metal forming and tape recorders [16], where the sheet metal or tape under tension is called a foil.
Active magnetic bearings also enable rubbing-free start/stops of the rotor, and the technology has been developed since early 90's as a magnetic-foil hybrid [17-19] or just magnetic bearing [20]. However, active magnetic bearings require complicated controllers, and full active control of a rotor with complicated dynamic motions is a significant challenge and real field applications are still very limited.
General interest in large AFBs (diameter>100 mm) has evolved from many gas-processing and military applications. The Mechanical Systems Branch within the Turbine Engine Division at Propulsion Directorate of US Air Force Research Laboratory (AFRL) has performed extensive research on AFB technology and its integration to high-speed engines for future aircraft and weapon systems [24].
Now referring to FIGS. 2A, 2B and 2C, a hybrid air foil bearing (HAFB) 200 is shown. The HAFB 200 includes a smooth top foil 102 disposed within a housing or bearing sleeve 106. The compliant structure is formed by twenty-four compression or coil springs 202 inserted within longitudinal bores 204 along the circumference of the interior wall 206 of the housing 106, such that a portion of the coil springs 202 extend slightly into the interior 208 of the housing 106 to support the top foil 102. A small stainless steel tube (hydrostatic air line) 210 with an inner diameter of approximately 1 mm is connected to each top foil orifice (air supply hole) 212 having a diameter of approximately 0.5 mm located in the middle of the top foil 102 via rubber tubes 214. The top foil orifices 212 are spaced apart from one another along the circumference of the top foil 102. As shown, the four hydrostatic air lines 210 are disposed within some of the longitudinal bores 204 and exit the housing 106. Details of the HAFB are disclosed by the following documents: (1) D. Kim and S. Park, “Hybrid Air Foil Bearings with External Pressurization,” ASME International Mechanical Engineering Congress and Exposition, ASME Paper IMECE2006-16151, (Nov. 5-10, 2006); (2) S. Park, “Hybrid Air Foil Bearings with External Pressurization,” Master of Science Thesis, Texas A&M University (May 2007); and (3) D. Kim and S. Park, “Hydrostatic Air Foil Bearings: Analytical and Experimental Investigation,” (Sep. 20, 2008), to be published in Tribology International, vol. 42, issue 3, pages 413-425 (March 2009).
The HAFB 200 combines benefits of air foil bearings and hydrostatic air bearings, providing very little friction during start/stops and stable operation due to damping of the elastic foundations. The hydrostatic air line 210 to the inside of the bearing clearance serves as effective energy dissipation mechanism via forced convective cooling of both the bearing and rotor surfaces. In addition, the HAFB 200 can eliminate the chronic wears of the top foil 102 and rotor 108 during startups and stops. The hydrostatic air serves as very effective cooling mechanism without any additional cooling air. Moreover, the HAFB 200 provides higher load capacity as compared to its hydrodynamic counterpart (with traditional cooling). The air flow rate used for hydrostatic operation is less than 10% of the cooling air flow used for hydrodynamic foil bearing. In addition, orbit simulations showed increased stability of the rotor bearing system due to external pressurization.
Despite these benefits and improvements, the HAFB (coil spring configuration) 200 is more expensive to manufacture than its inferior counterparts. Moreover, the one-to-one ratio of top foil orifices 212 to hydrostatic air lines 210, the connection of the hydrostatic air lines 210 to the top foil 102, and the exits for the hydrostatic air lines 210 through the housing 106 limit the effectiveness and durability of the HAFB 200. As a result, there is a need for a HAFB that is relatively inexpensive to manufacture and provides improved effectiveness and durability over current designs.